1. Field of the Invention
This invention relates generally to measuring and testing, and more specifically, to a constant frequency pulsed phase-locked loop measuring apparatus.
2. Description of the Related Art
A need exists to characterize states of metals and other materials and systems, for example, by measuring pressure derivatives of ultrasound phase velocities to determine combinations of third-order elastic constants, or by measuring stress derivatives in materials, or other quantities which undergo changes in ultrasonic phase velocities as a function of some thermodynamic variable or variables.
The pulsed superposition technique and the pulse echo overlap technique are generally known and used. Both measure true phase velocity by accurately measuring the time interval necessary for a tone burst to travel round-trip in the sample being measured.
Another system, known as the Blume technique, measures changes in frequency to maintain a "quadrature" condition as some thermodynamic variable changes, and is somewhat similar to a pulsed phase-locked loop. Two examples of a pulsed phase-locked loop are described in U.S. Pat. Nos. 4,363,242 and 4,624,142, both issued to Heyman and assigned to the National Aeronautics and Space Administration (NASA).
In U.S. Pat. No. 4,363,242, the radio frequency output of a voltage controlled oscillator (VCO) is periodically gated to a transducer which produces acoustic waves in a bolt. The reflected acoustic waves are converted to electrical signals by the transducer and gated to a mixer. The mixer also receives the output from the VCO and produces an output which is filtered by a low pass filter. The output of the low pass filter is a DC signal proportional to the phase difference change from a fixed phase difference between the two input signals to the mixer. The DC signal is then sampled at an instance and held by a circuit in response to a "P" signal (from a sample hold). The output of the circuit is integrated and then applied to the VCO to change the frequency of the VCO such that the phase difference between the two inputs to the mixer remains at the fixed phased difference. The frequency of the VCO is thus a measure of the change in strain of the bolt.
In U.S. Pat. No. 4,624,142, a double reference pulse phase locked loop measures the phase shift between the burst signals initially derived from the same periodic signal source, which is also a VCO, and delayed by different amounts because of two different paths. A first path is from a transducer to a front surface of the sample and back, and a second path is from the transducer to the rear surface of the sample and back. A first pulse phase locked loop including a phase detector and phase shifter forces the tone burst signals delayed by the second path in phase quadrature with the periodic signal source. A second pulse phase locked loop including another phase detector forces the tone burst signals delayed by the first path into phase quadrature with the phase shifted periodic signal source.
Some of the problems associated with the known techniques which measure phase velocity, such as the pulse-echo overlap method, is that it cannot be automated easily. Also, the pulse-superposition can cause high amplitudes of drive signal in order to obtain a measurement. In the pulseecho overlap method, it is necessary for an operator to estimate the overlap condition, while in the pulse-superposition method, the superposed pulses must be estimated by amplitude variation, and amplitude variations are not as sensitive to small changes as is phase comparison.
The Blume technique and the pulsed-phase-lock loop use phase comparison techniques, but change phase by changing frequency in order to obtain quadrature between the acoustic signal and the drive signal. This leads to a problem in that the consequences of changing frequency prevents one from measuring changes in true phase velocity and makes the measurement also sensitive to changes in transducer and bond characteristics. Moreover, the frequency change also makes non-contacting capacitive drive techniques incompatible with their operation.